The present invention relates to material processing and more particularly to an apparatus adapted for time averaging the intensity distribution of a beam of radiation.
Material processing applications typically require a rapid control scanning of a beam of high power radiation over the surface of a workpiece. Many heat treating applications utilizing laser radiation, such as transformation hardening of metal surfaces, require relatively precise control of the beam intensity incident onto the workpiece. Too intense a beam of radiation will produce melting of the surface material while too weak a beam produces insufficient depth of the transformation hardened surface to be of use. A beam of radiation having high power typically has an intensity cross-sectional distribution which varies considerably in the radial and azimuthal directions. Although the average intensity distribution may be adequate for the intended application, such as surface hardening, welding, etc., radiation hot spots and cold spots within the beam resulting from variations in the intensity distribution interact with the workpiece producing inhomogeneities in the interaction zone therein.
One method of time averaging the intensity distribution of a high power beam of radiation incident onto a workpiece is to oscillate the beam across the workpiece-radiation interaction zone. This method has been clearly demonstrated in electron beam technology wherein electron beams due to their electric charge can be readily scanned over an interaction zone with electric or magnetic means. Electro-optical and mechanical scanning means are available for low frequency oscillation of a beam of laser radiation typically having low power. However, low frequency oscillation of the beam to obtain time averaging of the intensity distribution does not improve material processing since the reaction time of the material is typically more rapid than the time required to scan the beam across the interaction zone and the beam material interaction retains the characteristics of that produced by a non-oscillating beam.
Stetson set al. in U.S. patent application Ser. No. 001,101 filed on even date herewith and held with the present application by a common assignee, discloses a mechanical scanning apparatus adapted for oscillating the focused zone of a beam of radiation having high power over an interaction zone on a workpiece to time average the intensity distribution of the radiation on the workpiece. The apparatus includes a compound beam adapted for being vibrated in a vibratory mode resulting in oscillatory motion of at least one end of the compound beam. A reflective surface fixedly attached to the end of the compound beam is adapted for focusing a beam of radiation incident thereon to a focused zone. Vibrating the compound beam in a fundamental vibration mode results in the end of the beam moving at the same frequency producing an oscillation of the focused zone. The rapid controlled scanning of the radiation across the interaction zone on the workpiece reduces the average intensity of the beam energy input onto the workpiece while maintaining a high instantaneous intensity on the interaction zone to promote effective coupling of the radiation with the workpiece. The apparatus is adapted for producing line scans and curvilinear scans of the focus zone to obtain radial and azimuthal time averaging of the intensity distribution of the radiation incident onto the workpiece. The present invention discloses a means of time averaging the intensity distribution of a beam of radiation incident onto a workpiece by rotating the beam about its optical axis or an axis substantially parallel thereto.